The invention relates to an inductive sensor, a position measuring system equipped with this sensor, and a method for its manufacture.
An inductive sensor is known, for example, from EP 0 743 508 A2, whose FIGS. 16A and 16B show a sensor in which a number of electrical conducting coils 102, 104 are applied to a substrate 126. The conducting coils are situated in a number of layers that are electrically separated from one another by thin insulating layers 162. An insulating layer 162 is also used to cover the sensor so that the conducting coils are protected from environmental influences, e.g. from the penetration of electrically conductive fluids. The substrate 126 is embodied in the form of an electrical printed circuit board. This substrate gives the whole arrangement the necessary mechanical strength. The conducting coil is manufactured by subjecting a thin copper layer, which is mounted over an entire area, to a photochemical etching process. The insulation layers 162 are varnish layers. FIG. 16b depicts the thickness of the conducting coils and insulation layers in an exaggerated fashion in comparison to that of the substrate.
A similar sensor is used in the linear position measuring device that is known from the publication “Integrated Measurement System Manual for Ball Tracks and Rolling Rail Guides” [Anleitung Integriertes Messsystem für Kugel-und Rollenschienenführungen], published by Bosch Rexroth AG (No. R310DE 2386; published April, 2004). To protect it from mechanical damage, this sensor was additionally covered with a thin steel plate. This metal cover is necessary because the above-mentioned guides are used, among other things, in machine tools in which they are exposed to corrosive coolants and lubricants. The thin protective layer according to EP 0 743 508 A2 would not be able to withstand such exposure. A cover made of a plastic would also be unsuitable since assuring the required durability would require it to be too thick. The cover plate is mounted after the contact has been produced between the conducting coil and the connecting line leading to the evaluation electronics since the contact point is no longer accessible afterward. In order to protect against the penetration of fluids, the sensor is also cast in a synthetic resin.
The steel plate cover attenuates the signals that the sensor generates when scanning the associated material measure. This attenuation is caused among other things by eddy currents that the alternating currents induce in the conducting coils of the sensor. These eddy currents can be minimized through the use of a particularly thin cover plate (thickness of 50 to 200 μm).
The signal is also attenuated by the increased distance between the conducting coils and the material measure. This distance is increased on the one hand by the thickness of the cover plate. The distance is also increased by the adhesive layer between the sensor and the cover plate. The distance is additionally increased by the covering insulation layer that is provided on the substrate during manufacture of the conducting coil. In this context, it should be noted that the overall distance between the conducting coils of the sensor and the material measure should under no circumstances be greater than the graduation of the material measure since otherwise, the necessary measurement precision is no longer assured. In the linear position measuring device mentioned above, the graduation is 1 mm; smaller graduations are being sought in order to increase the measurement precision. Where technically possible, the above-mentioned distance should be embodied to be as small as possible.
For the sake of completeness, reference is also made to DE 103 60 941 A1, which discloses a pressure sensor of a different generic type. In the embodiment according to FIG. 3, a measuring device 6 and a strain gauge 60 are applied to a diaphragm 4 using a thin-layer technology. Pressure sensors, however, are not comparable to the inductive sensors according to the present invention since the above-described distance problem does not arise in pressure measurement. The diaphragm disclosed also has to be a great deal thicker than the cover plate in an inductive sensor since it must withstand the compressive forces to be measured.